How do I close my Apple Card account

See the following from Legal - Data & Privacy - Apple

If you have questions about Apple Line of Credit, want to update your personal information, or to close your account, you can contact support by calling 877-255-5923. Support functionality by initiating Messages through Wallet app can be accessed after provisioning Apple Card to an eligible device.

This article may also help,

How to close your Apple Card account - Apple Support

You will have to confirm your account closing via confirming with the Apple Card Support Team at Goldman Sachs. If you follow the steps in the article you can cancel your account. You can also contact Goldman Sachs via the following methods,

CONTACTING — Goldman Sachs Bank 

• Using Messages; chat with an Apple Card specialist

• Call toll-free at 877-255-5923; or

• Write to: Goldman Sachs Bank, Lockbox 6112, P.O. Box 7247, Philadelphia, PA 19170-6112.